1. Field of the Invention
The present invention relates generally to noise trap or filter circuits, and more particularly to a noise trap circuit including signal blocking elements between input and output terminals of the circuit and shunt elements at the output terminals, to suppress small signal or noise components of a signal applied to the input terminals while allowing large signals to pass unattenuated from the circuit input to its output.
2. Description of the Known Art
Noise rejection circuits of various forms are known. For example, in electronic security systems, U.S. Pat. Nos. 3,828,337 (Aug. 6, 1974) and 3,863,244 (Jan. 28, 1975) disclose noise rejection circuitry which serves to discriminate true signals radiated from a resonant tag when the tag is moved through a surveillance zone of a retail store, from noise signals the presence of which bear no relation to the induced radiation from the tag. The disclosed noise rejection circuitry is provided in the security systems at a stage where pulses which may correspond either to true signals or noise are produced, and the circuitry employs pulse discrimination techniques to ensure that an alarm is sounded only in response to the true signal.
U.S. Pat. No. 4,019,148 (Apr. 19, 1977) discloses a circuit capable of detecting an AC data signal in the presence of high level noise signals of the kind produced by acoustical telemetry systems employed in bore holes. An unfiltered AC signal is placed across a pair of capacitors which are alternately switched to ground at a rate corresponding to the period of the AC component of the input signal.
A noise rejection circuit for use in sonar locator apparatus is disclosed in U.S. Pat. No. 3,747,053 (July 17, 1973). Like the mentioned '337 and '244 patents, the operation of the circuit in the '053 patent is based on a pulse processing scheme wherein pulses produced in response to electromagnetic or acoustical noise are discriminated and rejected, thus leaving only those pulses representing true signals or echos to be decoded.
Another sort of noise problem has arisen, however, in modern electro-acoustic systems which employ a common array of transducers for transmitting ultrasonic waves toward an object and receiving echo waves reflected by the object. When each transducer element of the array is driven with a large drive signal from a high power amplifier, it has been found that relatively small signals or noise produced at the outputs of the power amplifiers when in a receive mode, will appear at the input channels to the system receiver. Since the output noise from the amplifiers may be at a level within the dynamic range and bandwidth of the receiver and exceed the output levels of the transducer elements in response to normal "sea noise", it is essential to suppress any noise at the outputs of the system amplifiers well below the sea noise level when in a receive mode of operation.
An object of the invention is to provide a circuit that can discriminate between large and small amplitude signals, and the circuit must be capable of passing the large signals and rejecting the small signals. It must be capable of switching reliably and quickly between reject and pass modes of operation so that a transmit signal of short duration will be passed. Because of the reliability and timing considerations, mechanical relays would not be suitable for the application this invention was intended. Also, because it is necessary to reject noise at the transmit signal frequencies in the small signal/noise blocking mode of operation and then pass those frequencies in the large signal/transmit mode of operation, a frequency selective filter is not suitable.
A noise trap circuit according to the invention includes input terminal means to connect with an input signal from a signal source which produces large signals and noise comprised of relatively small signals, output terminal means, blocking means connected between the input and the output terminal means to establish a high impedance path to the noise while establishing a low impedance path to the large signals from the signal source, shunt means associated with the output terminals for establishing a shunt which substantially attenuates the noise when the shunt is in an on state, but which allows the large signals to appear unattenuated at the output terminals when the shunt means is in an off state, and means coupled to the shunt means for sensing the large signals and for maintaining the shunt means in the on state in the absence of the large signals, and for maintaining the shunt means in the off state when the large signals are sensed. Accordingly, the large signals from the signal source are provided at the output terminal means absent said noise.
In one embodiment, the noise trap circuit includes first and second input terminals, and first and second output terminals. Blocking means connected between the first input and the first output terminal establishes a high impedance path to small signal noise, and provides a low impedance path to large signals. Shunt means connected between the first and the second output terminals establishes a low impedance shunt to the noise when the shunt means is in an on state, and allows the large signals to appear unattenuated at the output terminals when the shunt means is in an off state. Feedback means coupled to the shunt means senses the large signals in an input signal applied to the first input terminal, and maintains the shunt means in the on state in the absence of the large signals. When the large signals are sensed, the feedback means maintains the shunt means in the off state.
According to another embodiment, the present noise trap circuit includes first and second input terminals, and first and second output terminals. First blocking means is connected between the first input terminal and the first output terminal, and second blocking means is connected between the second input and the second output terminals. Both of the blocking means are arranged to establish high impedance paths between the associated input and output terminals to small signal noise in an input signal applied to the first and the second input terminals, and to establish low impedance paths between the associated input and output terminals to large signals applied to the first and the second input terminals. Shunt means connected between the first and second output terminals establishes a low impedance shunt between the output terminals to the small signal noise when the shunt means is in an on state, and allows the large signals to appear unattenuated at the output terminals when the shunt means is in off state. Control means coupled to the first and the second blocking means and to the shunt means, senses the large signals at the input terminals and controls operation of both blocking means and the state of the shunt means, according to whether or not the large signals are sensed.
A low noise electro-acoustic system according to the invention includes a plurality of electro-acoustic transducers forming an array to transmit an ultrasonic wave toward an object, and to receive corresponding echo waves reflected from the object. A number of power amplifier means each supplies a large drive signal to a different one of the transducers when in a transmit mode. Each power amplifier means produces noise comprised of relatively small signals at its output when in a receive mode. Receiver means having an input coupled to the transducer array detects the echo waves and provides information corresponding to the object location when in the receive mode. A number of noise trap means each coupled between a different one of the power amplifier means and an associated transducer, suppress the noise produced by each amplifier means when in the receive mode from appearing at the input of the receiver means. Each noise trap means includes input terminal means for connecting with an output of an associated power amplifier means, output terminal means, blocking means connected between the input and the output terminal means, to establish a high impedance to the noise produced by the amplifier means and to establish a low impedance path for the drive signals from the amplifier means, shunt means to establish a low impedance shunt and attenuate the noise at the output terminal means when the shunt means is in an on state, and for allowing the drive signals to appear unattenuated at the output terminal means when the shunt means is in an off state, and means coupled to the shunt means for sensing the drive signals, and for maintaining the shunt means in the on state in the absence of the drive signals while maintaining the shunt means in the off state when the drive signals are sensed.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the present disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which there ar illustrated and described preferred embodiments of the invention.